Optical field curvature corrector

ABSTRACT

The invention is a device utilizing one or more carefully oriented optical prisms to alter the optical path length of a converging radiant-energy-ray bundle between the lens and the image plane of an optical system and to effect a focal correction in the information which may be collected through sequential scanning of the image plane.

Ullllefl otau 1111 3,720,454

Inderhees 1 1March 13, 1973 [5 1 OPTICAL FIELD CURVATURE 2,964,590 12 1960 Gillette ..350 1s7 CORRECTOR 3.378.687 4/1968 Schepler ..350/6 I 2,455,789 12 1948 Luboshez .352/116 i 1 lnvemorl John lnderhm, cmcm'lath Ohio 1,618,090 2/1927 Jenkins ..352/l16 [73] Assignee: Avco Corporation, Cincinnatti, Ohio I i Prtmary ExammerDav1d Schonberg Filed! y 1971 Assistant Examiner-Michael J. Tokar [211 App]' NOJ 143,533 Attorney-Charles M. Hogan 1571 ABSTRACT 52 U.S. c1 .350/7, 350/285 511 1111. c1. ..G02b 17/00 The "F""" 3 F 58 Field of Search ..350/6, 7, 43, 44. 134, 187, My wanted 29 Path 350/204, 285,287; 352/1 length of a converging radiant-energy-ray bundle between the lens and the lmage plane of an optlcal system and to effect a focal correction in the informa- [56] References Cited tion which may be collected through sequential UNITED STATES PATENTS scanning of the image P 1,356,787 10/1920 Porter ..352/l16 1 Claim, Drawing Figures --T a 'T 1 12 I \M 1o 1| SCAN MECHANISM BACKGROUND In prior art mechanical type scanning systems particularly where scanning is accomplished behind the lens or between the lens and focal plane, a change in optical path length occurs which causes undesirable defocussing. Various means and methods have been suggested to overcome this deficiency, but none has heretofore proved to be entirely satisfactory.

OBJECTIVES A major object of the invention is to overcome the difficulties which have heretofore been met in efforts to increase the effectiveness of scanning an optically created image and to provide a controlled pathstretching optical correction which alters the path length to produce an accurate focus on a plane which can be effectively scanned by conventional electrical pickup devices.

Another object is to provide a field curvature corrector which is simple in construction and easy and effective in operation.

Other objects and advantages of the invention will more clearly appear when reference is had to the following specification and accompanying drawings in which:

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagrammatic showing in elevation of a simple optical system utilizing a conventional scanning mechanism;

FIG. 2 is a diagrammatic illustration of how a scanning device normally produces a curved focal plane or surface.

FIG. 3 shows diagrammatically how a parallel plate optical element is introduced behind the lens and alters the converging rays to change the focus;

FIG. 4 illustrates diagrammatically the action of rotating wedges interposed between a scanning device and the focal plane of the optical system;

FIG. 5 is an end view of a rotating system according to FIG. 4 showing the direction of movement of the wedges and the geometry of their action; and

FIG. 6 is a diagram, in side elevation, of a focal plane which has been corrected in accordance with the invention.

DETAILED DESCRIPTION FIG. 1 depictsa conventional arrangement in which an optical scanner l2 ofthe mirror type, is interposed in the converging field of lens 10 for the purpose of depicting a scene on focal plane 11. In some applications the image depicted on focal plane 11 can be transformed into electrical pulses by means of photodiodes. In other applications, the image depicted on focal plane 11 may be recorded directly on photographic film. Use

of the focal plane data finds application in many instrumentalities in the control, display or entertainment fields. The scanning mechanism depicted in FIG. 1 operates as follows.

Front surfaced mirror 12 pivots on axis 23. A cam follower arm 24 attached to the back of the mirror mechanism, rides on off-centered cam 26, which is attached to the shaft of synchronous motor 25. Motor 25 Operationally, mirror 12 oscillates back and forth a few sits on base plate 28 which also serves to anchor axis 23 about which the mirror rotates. Spring 27 functions to make arm 24 remain in close contact with cam 26.

degrees whenever motor 25 is running. This oscillatory motion serves to cyclically scan a fieldof view to the left of lens 10 and image the scene on focal plane 11, which, for example, may contain photo sensors. The dashed lines a, b, c, d shown in FIG. I represent parallel rays of radiant energy entering lens 10 and converging along paths a, b, c, d to a focal point on image plane 11.

As shown in FIG. 2, the position of best focus in such a system shifts as a smooth function of the scan angle either towards or away from the lens as indicated by curve 13.

In FIG. 3 a parallel plate of transparent material having an index of refraction greater than air is introduced between the lens 10 and the focal plane, thus shifting the converging ray effectually so that it focuses at a point 15, more distant from the lens than otherwise would occur. This shift in focus is a function of the thickness T of the material 14 and its index of refraction n according to the following formula Correction of field curvature is shown in FIG. 4 in which two wedges l6 and 17 of suitable optical material are prepared so as to have a circular cross-section. They are axially mounted and introduced between the lens 10 and the focal plane 11 in overlapping relationship. By suitable means well known in the art, these wedges are mounted for rotation in the same direction about axes 16' and 17, the speed of rotation being mechanically synchronized with the scanning mechanism 12, as indicated by the dotted lines 18. Here the scan mechanism 12 is shown as being axially aligned with lens 10 and focal plane 11. This arrangement holds for the case where the scanning mechanism consists of a non-dispersive rotating cube of transparent material having an index of refraction different from that of air. Introduced in this manner, the two wedges vary their thickness in a periodic manner and the bending of the rays by each of the wedges individually is cancelled, since the bending by each wedge is in the opposite direction. It will be understood that the scanning mechanism may be of any conventional design which will sequentially scan the entire area of the focal plane and that the focal plane may be either axially aligned with the lens assembly or be offaxis as depicted in FIG. 1 for the case where a mirror scanner is used.

Referring to FIGS. 4 and 5 and defining a reference angle of B as zero when the two thin points of the wedges are aligned (as shown in FIG. 5) the combined thickness of the two wedges is:

when R, R,

T+ 2[T, aR aR cos B] where 1 is thickness of wedge in center a is a constant determined by thickness change R is the radius.

When the wedges rotate in the same direction B=a l and T=2[T, .+aR-aR cosmt] It can be shown that at any angle of prism rotation of the two prisms, the thickness is the same in the area of overlap between the two prisms.

FIG. 6 shows the scan-induced field curvature l3 and the wedge correction curvature 19, which, when applied, permits accurate scanning over the flat focal plane 11.

DISCUSSION It will be found that correcting the field curvature in accordance with the invention may not be entirely perfect, since the scan-induced field curvature and wedge correction are functions which are not necessarily always the same. However, the invention measurably improves the effectiveness of the scanning operation in equipment of this type. The invention permits the optical path length to lengthen or shrink sufficiently to accommodate off-axis scanning and maximizes the information available to the detectors at all angles.

A single rotating prism of circular cross-section can be used in place of the two prisms if the wedge can be placed very close to the focal plane and the angular deviations introduced by the wedge tolerated. In general, the corrections desired are small and the included angle between the faces of the prism is correspondingly small. In orienting prisms along an optical axis it is desirable to maintain the face of the prism on the side of the incoming radiant energy perpendicular to said optical axis. The face of the prism on the side of the departing radiant energy should likewise be maintained perpendicular to said axis.

It will be understood that the invention is not limited to an optical system for focusing light rays but may be adapted to a system in which infrared or other rays are focused on a screen for appropriate electronic pick-up.

Having thus described the invention, I claim as follows:

1. In an electro-optical scanning system the combination of a detector array formedwith a frontal plane surface and having a central axis normal to said array,

an optical lens having a focal point on said frontal plane,

a tilting ray-deflecting scanner interposed between sal lens and said detector array, said detector array and said optical lens and said tilting raydeflecting scanner being characterized by optical field curvature for rays oblique to said axis, and means for compensating for said optical field curvature away from said plane due to the tilt of said tilting ray-deflecting scanner, said compensating means comprising:

a pair of wedge shaped prisms individually mounted for rotation in the path between said tilting ray-deflecting scanner and said detector array,

said prisms overlapping at said central axis and being synchronized to rotate in the same direction at such angular position relative to the tilt of said tilting ray-deflecting scanner as to compensate for said field curvature,

the refraction due to one of said prisms being offset by the opposing refraction of a the other prism, the thickness of the two prisms together varying as a function of said angular position.

i t i W i 

1. In an electro-optical scanning system the combination of a detector array formed with a frontal plane surface and having a central axis normal to said array, an optical lens having a focal point on said frontal plane, a tilting ray-deflecting scanner interposed between said lens and said detector array, said detector array and said optical lens and said tilting ray-deflecting scanner being characterized by optical field curvature for rays oblique to said axis, and means for compensating for said optical field curvature away from said plane due to the tilt of said tilting ray-deflecting scanner, said compensating means comprising: a pair of wedge shaped prisms individually mounted for rotation in the path between said tilting ray-deflecting scanner and said detector array, said prisms overlapping at said central axis and being synchronized to rotate in the same direction at such angular position relative to the tilt of said tilting ray-deflecting scanner as to compensate for said field curvature, the refraction due to one of said prisms being offset by the opposing refraction of the other prism, the thickness of the two prisms together varying as a function of said angular position. 